The third generation partnership project, 3GPP, is currently working on specifying how HSDPA (High-Speed Downlink Packet Access) systems are to continuously develop in order to enable higher performance. The development includes several features for both UL (Uplink) and DL (Downlink) to enhance the system performance and the capacity as well as enabling a better user experience. Examples of developments are downlink MIMO (Multiple Input Multiple Output) (Release 7 of the 3GPP specifications) and dual cell/dual band HSDPA (Release 8 & 9). Currently in 3GPP radio access network working group RAN 2, a work item is ongoing to specify HSDPA Multiflow Data Transmission (MF-HSDPA) for Release 11 of the 3GPP specifications.
Hence, the concept of MF-HSDPA is to allow UEs (User Equipment nodes, also referred to as mobile/wireless terminals) to receive HSDPA data from two separate cells. The cells can belong to the same Node B, i.e. radio base station, (intra-site MF-HSDPA) or to different Node Bs (inter-site MF-HSDPA). In the former case, the solution is similar to DC-HSDPA (Dual Cell/Carrier HSDPA), but on the same frequency, with a data split in the MAC-ehs (Medium Access Control enhanced high speed) layer. In the inter-site case, the split may be in either the PDCP (Packet Data Convergence Protocol) or RLC (Radio Link Control) layer. Presently, the discussions are still ongoing of where to do the split. In FIG. 7, an illustration of inter-site MF-HSDPA is seen. The data is split in the RNC (Radio Network Controller), transmitted along the two links to the UE.
A potential benefit of introducing MF-HSDPA is that cell edge users may often suffer from bad coverage and/or low throughput which may bring down the overall system capacity. If these users could use available resources from neighbouring cells, i.e. receive data also from the non-serving cell, their situation could be significantly improved. This would improve the overall system capacity and the user performance for cell edge users.
As mentioned above, there are two alternatives of where to split the data between the links for inter-site MF-HSDPA, i.e., either split the data at the PDCP layer or at the RLC layer.
Communication links between a RNC and Node Bs are realized by way of the so-called lub interface. Data is communicated between the RNC and Node Bs using data frames carrying RLC PDU(s) (Protocol Data Unit(s)) encapsulated in Medium Access Control dedicated, MAC-d, PDU(s).
When RLC transmissions get stuck on one link, it may be a good alternative to retransmit the RLC PDU(s) (Protocol Data Unit(s)) over the other link, as illustrated in FIG. 8. If the retransmission cannot get through over the other link either, further retransmissions could be switched back to the original link. However, in this case there could be old copies of RLC PDUs existing at the link(s) besides the last retransmitted copy. This may cause certain problems. First, the redundant copies waste resources. The second issue relates to a case when the RLC PDU sequence number (SN) wraps around. The RLC receiver window may move forward when the retransmitted PDU is received at the UE. Since the other copies could take quite a while before arriving at the UE, the RLC SN could wrap around. When this happens and the delayed copy is eventually received by the UE, it may be treated as an original transmission. This may result in: (1) a lot of unnecessary retransmission due to the misjudged “missing SN”; and/or (2) the UE assembling the “old” retransmitted RLC PDUs with this misjudged new data causing “corrupted RLC SDUs” to be delivered to higher layers.